Note: Descriptions are shown in the official language in which they were submitted.
W095132797 _ ~~ PCTIUS95I06323
ABSORBENT AND PROCESS FOR REMOVING
SULFUR OXIDES FROM A GASEOUS MIXTURE
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention relates to anionic, hydrotalcite-type pillared clay
compositions and their heat-treated derivatives. The invention also relates
to a process for reducing the sulfur oxide content of a gaseous mixture by
absorbing sulfur oxides on an absorbent which can be reactivated for further
absorption through contact with a hydrocarbon in the presence of a
hydrocarbon cracking catalyst.
II. Description of the Prior Art
The development of efficient methods and catalysts for reducing the
concentration of air pollutants, such as sulfur oxides, in gaseous mixtures
which result from the processing and combustion of sulfur-containing fuels
presents a major industrial problem which has interested researchers for a
considerable time. For example, U.S. Patent No. 3,835,031, issued to
Bertolacini et al, and assigned to the assignee of the present application,
describes a cyclic, fluidized catalytic cracking process operating with a
catalyst comprising a molecular sieve in a silica-alumina matrix which is
impregnated with one or more Group IIA metal oxides, such as magnesium
oxide. By absorbing sulfur oxide within a regeneration zone and,
subsequently, releasing the absorbed sulfur within a cracking reaction zone,
emission of sulfur oxides in a regenerator stack gas stream is greatly
reduced.
Other researchers have noted that absorbents containing rare earth
metals are suitable for sulfur oxide removal service. U.S. Patent No.
4,146,463, issued to Radford et al. and assigned to the assignee of the
present invention, describes the absorption of sulfur oxides by modified
catalyst particles containing the oxides of rare earth metals, such as cerium,
lanthanum and neodymium. The modified catalyst particles reportedly form
non-volatile sulfur compounds by reacting with sulfur oxides in a
regeneration zone.
Researchers have attempted to identify an optimal structure for sulfur
oxides separation catalysts. U.S. Patent No. 4,626,419, issued to Lewis et
VJO 95132797 ~ 19 0'~ 4'~ PCT/US95106323
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al., is directed to a composition of matter for removing sulfur oxides from
gases which comprises an alkali metal and a crystalline rare earth oxide,
such as cerium oxide, having a crystal size of less than about 90 Angstrom
units. The '419 Patent states that improved results measured as a reduction
of sulfur in regenerator off-gas may be ota~ned using oxide crystals in the
specified size range. ~~'~m
Sulfur oxide separation cats containing magnesium and
aluminum crystalline structures in spmel form are reported, for example, in
U.S. Patent No. 4,790,982, issued to Yoo et al., which describes the use of a
magnesium and aluminum spinal in conjunction with cerium metal and free
magnesia. U.S. Patent No. 4,728,635, issued to Bhattacharyya et al., is
directed to a process for the production of a calcined alkaline earth,
aluminum-containing spine( composition for use as a sulfur oxide and
nitrogen removal agent.
U.S. Patent No. 4,865,019, issued to Van Broekhoven, describes
sulfur-oxide absorbents which comprise an anionic clay having a
hydrotalcite structure. The '019 Patent states that the anionic clay can have
a layered structure corresponding to a formula calling for divalent rations,
trivalent rations, and anions in specified proportions. Preference is given to
divalent rations Mg2+ and trivalent ration AI3+ alone or combined with La3+
and/or Ce3+. Anions N03_, OH-, CI-, Br-, I-, C032-, S042-, Si032-, Si032-
Cr042-, HP042-, Mn04-, HGa032-, HV042-, CI042-, BOgz-,
monocarboxylates, dicarboxylates, alkyl sulfonates, and combinations
thereof are listed as suitable. The '019 Patent states that the absorbents are
useful after a heat treatment to a temperature in the range of about 300 to
about 900°C which reportedly can involve some decomposition of the
hydrotafcite structure.
U.S. Patent No. 4,774,212, issued to Drezdon and assigned to the
assignee of the present invention, describes magnesium and aluminum
hydrotalcite-type clay compositions having polyoxometalates of vanadium,
tungsten, or molybdenum as pillaring anions. The compositions are
reported to have an x-ray diffraction d(003) value which is larger than that
of
typical hydrotalcites, indicating a greater spacing between clay layers.
Reference is made to use of the compositions for catalysis at temperatures in
the range of about 200 to about 600°C. The '212 Patent also presents a
method of preparing the described compositions which involves formulating
hydrotalcite-like clays pillared by relatively large organic anions and
PCTIUS95106323
W O 95132797
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replacing the organic anions with polyoxometalates from a solution having a
pH of about 3 to about 6.
A direct and relatively simplified process for making anionic clays
having a hydrotalcite structure pillared by pH-dependent inorganic anions is
set forth in U.S. Patent No.'~5,246,899, issued to Bhattacharyya and assigned
' to the assignee of the present invention. In a preferred aspect, the process
comprises adding a solution containing one or more selected divalent
cations and one or more selected trivalent cations to an essentially
carbonate-free solution which includes an inorganic ion and has a
selectively chosen pH between about 6 and 14.
U.S. Patent No. 5,288,675, issued to Kim, contemplates a
MgOILa20s/A1203 ternary oxide base wherein the Mg0 component is
present as a microcrystalline phase which may be detected by x-ray
diffraction. The ternary oxide base can reportedly be used in combination
with ingredients such as ceria and/or vanadia to control sulfur oxide
emissions. The '675 Patent states that the combination can be prepared by
a multi-step process which includes reacting an aged, coprecipitated
lanthanum and aluminum hydrous oxide slurry with a magnesium oxide
slurry and a sodium hydroxide solution, calcining, impregnating with
solutions of cerium andlor vanadium and calcining at a temperature of
450°
to 700°C.
Sulfur oxide emissions from fluid catalytic cracking units, for example,
are increasingly restricted by environmental regulations. The removal of
sulfur oxide pollutants has been the subject of considerable attention for
several years. One approach to reducing such emissions involves
desulfurizing a hydrocarbon feed stream before it enters the cracking unit, so
that a lesser amount of sulfur oxides are produced. Another approach is to
scrub the emissions stream with an inexpensive alkaline material, such as
lime or limestone. However, both of these approaches are relatively
cumbersome and they create other waste disposal problems. Accordingly,
separating the sulfur oxides by contact with a reusable absorbent presents
an appealing alternative.
It is generally accepted that sulfur trioxide (S03) absorption proceeds
more rapidly than sulfur dioxide (S02) absorption. Accordingly, efficient
sulfur dioxide absorbents must perform at least three functions. First,
desirable absorbents have a catalytic capability that allows them to enhance
the reaction of sulfur dioxide with oxygen to form sulfur trioxide. Second,
desirable absorbents are capable of binding sulfur trioxide in relatively
large
219075 _
VVO 95f32797 PCTIUS95106323
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amounts. Third, desir~a'~:~~~ bents can desorb sulfur components
comparatively quickly on exposure to hydrocarbons and cracking catalyst.
The sulfur oxide absorbents which have received the widest
commercial acceptance to date in fluidized catalytic cracking units are based
on spinet technology, most notably MgAl20ø spinets combined with cerium
oxide. Although the spinet and cerium absorbents are adequate for many
purposes, they exhibit limited absorbent capacity and are prone to
deactivation. In particular, free cerium oxide crystals present in the spinet
and cerium absorbents tend to increase in size during normal operation so
as to inhibit overall activity. Additionally, the spinet and cerium absorbents
require more time for complete desorption than is available in some cyclic
processing schemes.
Accordingly, a need still exists for new absorbents which can absorb
and desorb comparatively larger amounts of sulfur compounds per unit
mass within relatively short cycle time periods. Catalytic materials on the
absorbents must be well dispersed for maximum accessibility and resist the
tendency to agglomerate under operating conditions. Additionally, the
absorbents should resist physical attrition and demonstrate superior stability
at processing temperatures in both oxidizing and reducing environments.
SUMMARY OF THE INVENTION
The invention is an improved absorbent composition composed
substantially of relatively small microcrystallites which demonstrate
desirable sulfur oxide absorption capacity and comparatively fast absorption
and desorption rates. High resolution electron microscopy reveals that a
substantial portion of the microcrystailites are essentially composed of a
solid solution having impurities, such as aluminum oxide, dispersed in a
monoxide of a divalent metal. The improved absorbent also includes spinet
microcrystallites and trivalent metal oxide microcrystallites. The improved
absorbent can be produced by heat treating layered mixed hydroxide
compositions having interlayer anions in monometalate, dimetalate,
trimetalate, or tetrametalate form. The invention is also a sulfur oxide
separation process which exploits the advantages of the improved
absorbent.
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In a preferred aspect, the invention is an anionic layered mixed
hydroxide composition having the formula:
2+ 3+ ~ ~~ k ~ q1 2~k mH20
M2x Al2-n M n ~~H~4x+4
where M2+ is a divalent metal selected from the group consisting of
magnesium, calcium, zinc, barium, and strontium. M3+ is a trivalent metal
ration selected from the group consisting of cerium, lanthanum, iron,
chromium, vanadium, and cobalt.
J IS V03, HV04, V04, V207, HV207, V30g, V40~2, W04 Or Mo04. A IS
selected from the group consisting of C03, OH, S03, S04, Cl, and N03. q
and v are the net ionic charges associated with J and A, respectively. x is
about 1.1 to about 3.5, while n is about 0.01 to about 0.4, m is a positive
number.
In another preferred aspect, the invention is a composition suitable for
use as a sulfur oxide absorbent. The absorbent comprises microcrystallites
collectively of the formula:
2+ 3+
M AI M T O
2m 2-p P r 7+rs
where M2+ is a divalent metal, and M3+ is a trivalent metal, as
described above. T is vanadium, tungsten or molybdenum. p is about 0.01
to about 0.4, while r is about 0.01 to about 0.2. s is 2.5 when T is vanadium
or 3 when T is tungsten or molybdenum. Each of the microcrystallite has a
greatest linear dimension in the range of about 0.1 to about 30 nanometers.
Moreover, a substantial portion of the microcrystallites of the invention are
essentially composed of a solid solution phase having impurities, such as
aluminum oxide or vanadium oxide, dispersed in a monoxide of the divalent
metal. Another portion of microcrystailites are essentially composed of a
spinet phase.
In yet another preferred aspect, the invention is a process for
manufacturing a composition suitable for use as a sulfur oxide absorbent. In
the process, water is blended with about two molar parts of a salt of a
divalent metal selected from the group consisting of magnesium, calcium,
zinc, strontium, and barium. Additionally, one molar part in sum of an
aluminum salt and a selected trivalent metal is also blended in the mixture.
.' ~ ~~ ~~-~ N,
219f074~
W095132797 ' PCT1US95/06323
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The trivalent metal is selected from the group consisting of cerium,
lanthanum, iron, chromium, vanadium and cobalt. A metalate salt in a
quantity of about 0.1 to less than 2 molar parts is also blended into the
mixture. The metalate salt contains an anionic species which is a vanadate, '
a tungstenate, or a molybdenate. In an especially preferred aspect, an
alkalinity control agent is also blended into the mixture to stabilize the
anion
in an aqueous, dissociated form which contains but a single metal atom.
Anions in such form are commonly described as monometalate. It is
sometimes convenient to introduce the anions in solution form as a separate
step after a precipitate has formed and has been calcined at least once.
The mixture is heated to a temperature in the range of about 50° to
about 100°C for at least about one hour and, thereafter, an anionic
layered
mixed hydroxide is recovered from the mixture. The recovered layered
hydroxide is calcined for about one hour at a temperature of about
450°C or
hotter to produce a collapsed composition suitable for use as a sulfur oxide
absorbent. The collapsed composition is substantially composed of
microcrystallites having a greatest linear dimension in the range of 0.1 to
about 30 manometers. The collapsed composition contains microcrystallites
which are essentially composed of a solid solution phase having aluminum
oxide or vanadium oxide dispersed in a monoxide of the divalent metal. Ths
collapsed composition also contains microcrystallites which are essentially
composed of a spinet phase.
In an additionally preferred aspect, the invention is a process for
manufacturing a composition suitable for use as a sulfur oxide absorbent
which comprises calcining a layered mixed hydroxide at a temperature of at
least about 450°C for about one hour or more. The layered hydroxide is
of
the formula set forth above in regard to layered mixed hydroxides of the
invention. The product of calcining is a collapsed composition substantially
composed of microcrystallites, each of about 0.1 to about 30 manometers in
size. The microcrystallites are constituted by solid solution phase
microcrystallites and by spinet phase microcrystallites, as described above.
Moreover, the invention is a process for separating sulfur oxides from
gaseous mixtures. The process comprises absorbing sulfur oxides on a
dehydrated and collapsed composition which is substantially composed of
microcrystallites collectively of the formula set forth above with regard to
collapsed compositions of the invention. The microcrystallites are of about
0.1 to about 30 manometers. One portion of the microcrystallites is
constituted by a solid solution phase and another portion of the
;y1 9 ~#?_~ 5
WO 95132797 - - 7n_; ..' t y1 ~' ~ ~j PCT/US95106323
microcrystallites is constituted by a spinet phase, as described above. The
process additionally comprises desorbing sulfur dioxides from the absorbent
by contacting the collapsed composition with a hydrocarbon in the presence
of a cracking catalyst.
The invention is also a process for the cyclic ffuidized catalytic
cracking of a hydrocarbon feedstock containing organic sulfur compounds.
The process comprises absorbing at least a portion of the sulfur oxides in a
regeneration zone with a fluidizable particulate solid including a collapsed
composition in accordance with the formula set forth above. The collapsed
composition includes microcrystallites composed essentially of a solid
solution phase having aluminum oxide dispersed in a monoxide of a
divalent metal, as described above. The process further includes removing
absorbed sulfur oxides from the particulate solid by exposing the particulate
solid to the hydrocarbon feedstocks in the reaction zone.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is the trace of an x-ray diffraction analysis for the product of
Example 1, showing a relationship between intensity and an angle (in
degrees) that is characteristic of a mixed layered hydroxide structure having
a d(001 ) value of 7.62 Angstroms units;
FIG. 2 is a dual-axis graph showing a relative weight for the product of
Example 2 as a function of time (in minutes) during the course of a thermal
gravimetric analysis having a temperature trace which is also presented in
FIG. 2;
FIG. 3 is a dual-axis graph which depicts a relative weight for the
product of Example 3 as a function of time during the course of a thermal
gravimetric analysis having a temperature trace which is also presented in
FIG. 3; and
FIG. 4 is a dual-axis graph exhibiting a relative weight trace and a
temperature trace for a second cycle and a third cycle of thermal gravimetric
analysis pertormed on the product of Example 3.
DETAILED DESCRIPTION OF THE INVENTION
We have discovered that anionic layered mixed hydroxide
compositions can be calcined to produce dehydrated and collapsed
compositions having unique solid solution microcrystallites that are very
suitable for use as sulfur oxide absorbents. The layered compositions will
be described first.
~19074a
W 0 95132797 PCTIUS95I06323
Anionic layered' mixed h~,droxide compositions are layered in the
sense that they are co~n,~tii~fiy sheets of divalent and trivalent metal
cations interposed between a larger number of hydroxide anions which are
also in the sheets. They are mixed because divalent cations and trivalent
cations are interspersed within the sheets. Water molecules and additional
anions are located in interlayers between the sheets. When subjected to
conventional x-ray diffraction analysis, the layered mixed hydroxide
composition exhibits diffraction peaks whose positions can be correlated
with the average distance between adjacent sheets.
In one aspect, the invention is an anionic layered mixed hydroxide
composition of the formula:
MZX AI2.n M~+t;OH)4x+4 ~dv~ k CAq) Za' ' mH20
v 4
In the formula, M2+ is a divalent metal present as a cation having a
valence of positive two which is coordinated with a plurality of hydroxide
anions to form infinite sheets. The structure of the sheets resembles the
structure of the naturally occurring mineral brucite, Mg(OH)Z. The divalent
metal is preferably selected from the elements of Group IIA or IIB of the
periodic table of the elements as depicted on the inside front cover of
Perrv's
Chemical Enr~ineers Handbook (6th Edition). More preferably, the divalent
metal is selected from the group consisting of magnesium, calcium, zinc,
strontium, and barium; most preferably from the group consisting of
magnesium, calcium, and zinc.
The symbol M3+ denotes a trivalent metal present as a cation of
valence three. The trivalent metal is preferably selected from the group
consisting of rare earth elements, iron, chromium, vanadium, and cobalt.
More preferably, the trivalent metal is selected from the group consisting of
cerium, lanthanum, iron, chromium, vanadium, and cobalt; most preferably
cerium and lanthanum. The rare earth elements, also known as the
lanthanide series elements, are often found as a naturally occurring mixture
of two or more of the rare earth elements. It is contemplated that such
mixtures of rare earth elements may be conveniently employed in the
present invention.
x is about 1.1 to about 3.5; preferably about 1.5 to about 3.5; and
more preferably about 2 to about 3. n is about 0.01 to about 0.4, more
preferably about 0.2 to about 0.4. AI is the trivalent metal aluminum present
W 0 95/32797
PCT/US95106323
as a trivalent cation coordinated with a plurality of hydroxide anions.
Additionally, it is preferred that the molar ratio of the divalent metal to
aluminum be about 1 to about 5, more preferably about 2 to about 3.
J is an anion selected from the group consisting of vanadates,
tungstenates and moiybdenates. Preferably, J is V03, HV04, V04, V20~,
HV207, V309, V4012, W04 or Mo04. More preferably, J is selected from the
group consisting of monovanadates, specifically metavanadate (V03),
orthovanadate, (V04) and protonated vanadate (HV04). Preferably, J is
located primarily in interlayers between the sheets. v is the net anionic
charge associated with J. For example, the net ionic charge associated with
V03~- is -1. Similarly, the net ionic charge associated with HV042- is -2.
A is a relatively small anion having a size approximately equal to or
less than that of carbonate (C03). Preferably, A is selected from a group
consisting of C03, OH, S03, S04, CI, and N03. q is the net ionic charge
associated with A. k is about 0.01 to less than 2.
m is a positive number quantitatively indicating the presence of water
molecules. Preferably, substantially all of the water molecules are located in
the interlayer. However, the formula presented is empirical and is not limited
to any particular structure.
By way of comparison, the naturally occurring mineral hydrotalcite is a
specific example of an anionic layered mixed hydroxide composition.
However, the formula of hydrotalcite differs from the formula of the layered
composition of the present invention in that the mineral hydrotalcite
ordinarily contains substantially no divalent metals other than magnesium,
substantially no trivalent metals other than aluminum, and substantially no
anions other than carbonate.
The layered composition of the present invention exhibits an x-ray
diffraction pattern when analyzed using conventional techniques, preferably
an x-ray diffraction pattern including a d(001 ) value equal to or greater
than
about 7.6 Angstrom units. An especially preferred layered composition
employs magnesium as the divalent metal, cerium as the trivalent metal, and
a monovanadate as the anion J.
In another aspect, the invention is a dehydrated and at least partially
collapsed composition suitable for use as a sulfur oxide removal catalyst.
Preferably, the collapsed composition is prepared by heat treating the
layered composition described above. Regardless of its source or method of
preparation, the collapsed composition comprises microcrystallites which
are collectively of the formula:
W095132797 .' ~,~ ~~J ~ PCTIUS95106323
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2+ 3+
M2m A12-p M P T~ 07+rs
In the formula, MZ+ is a divalent metal, AI is aluminum and M3+ is a
trivalent metal, as described above. T is vanadium, tungsten or
molybdenum, preferably vanadium.
m is preferably about 1.1 to about 3.5, more preferably about 1.5 to
about 3.5, and most preferably about 2 to about 3. p is about 0.01 to about
.4, preferably about 0.2 to about 0.4. It is preferred that the molar ratio of
the
divalent metal to aluminum be about 1 to about 5, more preferably about 2 to
about 3.
r is about 0.01 to about 0.2, preferably about 0.05 to about 0.2. In the
algebraic expression communicating the subscript for O in the above
formula, r is multiplied times s. s is 2.5 when T is vanadium, but s is 3 when
T is tungsten or molybdenum.
Each of the microcrystallites of the invention has certain identifying
characteristics. It is not necessary that every microcrystallite present in a
composition possess these characteristics, but a substantial proportion of
the microcrystallites must possess the characteristics in order to achieve the
advantages offered by the invention.
First, the microcrystallites of the invention have a greatest linear
dimension in the range of about 0.1 to about 30 nanometers, more
preferably about 1 to about 20 nanometers and most preferably about 10
nanometers. The microcrystallites need not be spherical. For
microcrystallites which are spherical, the greatest linear dimensions are the
diameters.
Absorbents having microcrystallites with dimensions in the specified
range are sometimes termed "amorphous," possibly because conventional
x-ray diffraction analysis techniques are inadequate to detect the presence
of their relatively small crystal structure. However, high resolution electron
microscopy routinely detects microcrystallites in this size range. For the
present purposes, high resolution electron microscopy is defined as electron
microscopy capable of a point-to-point resolution of at least about 2.0
Angstrom units.
Such electron microscopy is also capable of detecting lattice planes
in microcrystallites. A lattice plane is a regular geometrical arrangement of
objects in space, such as atoms arranged in a crystalline structure, that is
relatively flat in a given vicinity. When viewed on edge by appropriate
W095132797 ' ' ~ ~ PCTIUS95106323
electron microscopy techniques, the lattice planes appear as lines which
can be curved or straight as well as continuous or discontinuous.
Additionally, the electron microscopy techniques can pinpoint the
existence and location of individual lattice planes, the relative intensity of
various lattice planes, and the spacing between adjacent lattice planes.
' Taken together, these observations of the crystal lattice, termed "lattice
parameters," can be used to distinguish between two or more phases within
a high resolution electron microscopes field of view. Herein, a phase is
understood to be a homogeneous, physically'distinct portion of matter
present in a non-homogeneous physical-chemical system.
In practice, the lattice planes exhibit identifying lattice parameters,
such as spacing, relative intensity, and periodic repetitions in spacing and
intensity, which can be utilized to distinguish between phases. Once the
presence of distinguishable phases has been determined, it may be
necessary to carry out other types of analyses in order to precisely
determine compositions of the phases. For example, scanning electron
microscopy is often used to confirm the phase compositions.
Secondly, a substantial portion of the microcrystallites of the present
invention are essentially composed of a solid solution phase in which an
impurity, such as aluminum oxide (AIp03) or vanadium oxide, is dispersed
within a crystal lattice of a monoxide of a divalent metal, such as magnesium
oxide (Mg0). The dispersion is not merely a physical aggregation. Rather,
the impurity is present as a dopant. The impurities are located so as to
expand the crystal lattice of the divalent metal monoxide but not to disrupt
the crystal lattice completely.
Another portion of the microcrystallites is composed essentially of a
spinal phase. For example, the spinet phase may include a magnesium
spinal such as MgA1204. Additionally, microcrystallites composed
essentially of an oxide of the trivalent metal are preferably present.
The solution solid phase having aluminum oxide dispersed in a
divalent monoxide crystalline structure is considered to be highly unusual. It
is much more common to find relatively separate microcrystallites of
aluminum oxide and the divalent monoxide together, with each
microcrystallite containing but a single oxide. Accordingly, it is
hypothesized
that the presence of the solid solution phase of the present invention
indicates a tendency to resist further division between the two phases. It is
believed that the presence of the solid solution phase correlates with
improved stability and activity under hydrocarbon processing conditions.
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The divalent,i~a~t~~~~d phase can be selected from the Group IIA
and Group IIB elements. Preferably the divalent metal is selected from the
group consisting of magnesium, calcium, zinc, strontium, and barium, more
preferably from the group consisting of magnesium, calcium and zinc. It is
especially preferred that the divalent metal oxide phase be composed
essentially of magnesium oxide.
The spinet phase is constituted by elements in crystalline spinet form,
although the spinet may be present as microcrystallites too small to be
detected by conventional x-ray diffraction analyses. The spinet structure is
based on a cubic close packed array of oxide ions. Typically, the crystalline
unit cell of the spines structure contains 32 oxygen atoms. With regard to
magnesium aluminum oxide spinet, there are 8 magnesium atoms and 16
aluminum atoms in each unit cell, corresponding to the formula MgAI204.
If oxide crystals of the trivalent metal described above are present,
such as crystals of cerium oxide or lanthanum oxide, the trivalent metal
oxide crystals each must be substantially in the form of microcrystallites
having a greatest linear dimension in the range of about 0.1 to about 30
nanometers, preferably about 1 to about 20 nanometers. It is especially
preferred that the trivalent metal be predominantly situated in relatively
homogeneous microcrystallites of the present invention coexisting with solid
solution microcrystallites and spine) microcrystallites.
It is hypothesized that the presence of the trivalent metal oxide phase
with the other microcrystallites of the present invention provides a desirable
degree of dispersion for the trivalent metal atoms and also tends to protect
trivalent metal oxides from attrition. Cerium oxide crystals standing alone,
for example, have a tendency to disintegrate under the stresses of fluidized
bed processing.
In another aspect, the invention is a process for mahufacturing a
composition suitable for use as a sulfur dioxide absorbent. A mixture is
produced by blending water with about two parts by moles of a salt of a
divalent metal selected from the group consisting of magnesium, calcium,
zinc, strontium, and barium; preferably magnesium. Additionally, about one
part by moles of the combined sum of an aluminum salt and a salt of a
trivalent metal selected from the group consisting of cerium, lanthanum, iron,
chromium, vanadium and cobalt is blended into the mixture. The sum is
calculated by adding the molar amount of the aluminium salt to the molar
amount of the trivalent metal salt and dividing the total by one-half of the
molar amount of the divalent metal salt. Also blended in the mixture is about
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0.01 to less than 2 molar parts of a metalate salt. The metalate salt of the
present invention contains an anion which is a vanadate, a tungstenate or a
molybdenate. Herein, salt is intended to mean a cation and an anion joined
in an ionic bond. For convenience, we- refer to solutions containing
dissolved salts as salt solutions even'though the ionic bonds have become
dissociated.
!n a preferred aspect, the process also includes blending into the
mixture an appropriate amount of an alkalinity control agent to stabilize the
anions in an aqueous, disassociated metalate form selected from the group
consisting of monometalate, dimetalate, trimetalate, and tetrametalate
anions. In an especially preferred aspect, the anions are monorrietalates
which each contain exactly one metal atom. For example, V03~-, HV04 2;
and V043- represent anions which each contain exactly one metal atom.
Allowance must often be made for losses of the alkalinity agent to
precipitation. Stabilization of,the metalate anion typically takes place in a
.
liquid phase portion of the blended mixture.
For the present purposes, blending is understood to include methods
wherein aIJ of the described ingredients are blended simultaneously, and
also to include methods wherein two.or more of the ingredients~are blended
with each other and then blended with other ingredients. At each
successive combination of ingredients, care must be .taken to provide
concentrations and alkalinities which tend to precipitate desired divalent
metals and trivalent metals, as described above. The aIkaIJnity of the
mixture can also determine which forms of the anions are stabilized in
~ solution and ultimately become components of the precipitates. It is often
convenient to delay addition of the metallate anions until after a precipitate
has formed and has been subjected to recovery and calcining.
The final choice of blending amounts and conditions is guided by the
knowledge of previous practitioners in the art. For example, U.S. Patent No.
5,246,899 and U.S. patent No. 5,354,932
-contain useful teachings tegarding
pH-dependent anions and intercalating agents. Additionally,. pages 181
through 182 0f "The Early Transition Metals,'" by D.L.. Kepert, Academic
Press (New York) are recommended for information on stabilizing vanadate
ions in solution.
In an especially preferred aspect of the process, the metalate salt
contains a vanadate and is blended in an amount sufficient to produce a
concentration of the vanadate in a liquid phase of the mixture which is in the
W095132797 X190745_
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range of about 0.01 to about 1 molar. Additionally, sufficient alkalinity
control agent resides in the liquid phase of the mixture to produce an
alkalinity in the range of about 6 to about 14 pH. The range of about 6 to
about 8 pH is appropriate for VOg~-. The range of about 9 to 13 pH is
appropriate for HV042-. The range of about 13 to about 14 is appropriate for
V043-. '
In this especially preferred aspect of the process, it is contemplated
that about 50 percent or more of the vanadate ions dissolved in the liquid
portion of the mixture are in a monometalate vanadate form, such as HV042-
, VO43- or V03~-. Because the metavanadate anion (V03~') and the
orthovanadate anion (V043-) are stabilized at conditions which are
inconsistent with the precipitation of many divalent metal hydroxides and
trivalent metal hydroxides, it is often convenient to blend solutions
containing these anions with the product of the recovery and the first
calcination described above. Preferably, the product is calcined again after
being exposed to the metalate salt solution.
The heated mixture is permitted to stand for at least about 1 hour,
preferably at a temperature in the range of about 50° to about
100°C, more
preferably about 75° to 100°C. Higher temperatures may be
utilized
provided that commensurately higher pressures exist to maintain at least a
portion of the mixture in the liquid phase. An anionic layered mixed
hydroxide, as described above, is recovered as a solid from the mixture.
Appropriate recovery techniques include but are not limited to filtration,
evaporation, fractional crystallization and spray drying.
The recovered layered mixed hydroxide is calcined for not less than
about 1 hour at a temperature of about 450°C or hotter. Preferably the
temperature is in the range of about 450° to about 1000°, more
preferably
about 450° to about 850°C. Herein, calcining refers to the
heating of a solid
in the presence of a gas, preferably a flowing gas. The gas may be air or,
alternatively, a relatively inert gas, such as nitrogen, helium, or carbon
dioxide. The recovered material may be calcined more than once,
preferably before and after one or more exposures to a metalate salt solution
as described above. The final calcining produces a dehydrated and
collapsed composition suitable for use as a sulfur oxide absorbent.
The collapsed composition is dehydrated in the sense that at contains
essentially no associated water molecules. Additionally, the sheets
containing metal cations coordinated with hydroxide anions, as described
above, are at least partially disrupted by the calcining and are in a
condition
.} , .., ,_ a
W O 95132797
.~ ~; r pCTIUS95106323
=-1 5--
conveniently termed "collapsed." Rather than having sheets of 400
nanometers or more in length as are typical of the layered mixed hydroxides,
the collapsed composition is substantially composed of microcrystallites,
each having a greatest linear dimension in the range of about 0.1 to about
30 nanometers. It is hypothesized that the microcrystallites are disintegrated
and jumbled remnants of the layered mixed hydroxides which are believed
to have at least partially collapsed when subjected to the calcining.
Repeated calcination steps interspersed by contact with aqueous solutions
are believed to repeatedly disrupt and reform the layered sheets so as to
produce a final collapsed product having an excellent degree of
microcrystallite dispersion.
In any event, the microcrystallites of the invention are relatively small
and well dispersed so as to be accessible to reactants such as sulfur oxides.
A substantial portion of the microcrystallites are constituted by a solid
solution phase having aluminum oxide dispersed in a divalent metal
monoxide. Another portion is constituted by a spine) phase. The phases
contain lattice planes discernible by high resolution electron microscopy.
In yet another aspect the invention is a process for manufacturing a
composition suitable for use as a sulfur oxide absorbent which comprises
calcining a layered mixed hydroxide of the formula:
2+ 3+ ~ v\ g ~ q\ 2-k mH20
MZx AI2-n M n ~OH~4x+4 ~ J A
v q
M2+ is a divalent metal selected from the group consisting of
magnesium, calcium, zinc, barium, and strontium. Divalent metals selected
from the group consisting of magnesium, calcium, and zinc are preferable
because in practice they more readily form sheets of metal cations
coordinated with hydroxide anions, as described above. Magnesium is
especially preferred as the divalent metal.
x is about 1.1 to about 3.5, preferably 1.5 to 3.5; n is about 0.01 to
about 0.4, preferably about 0.1 to about 0.3. The trivalent metal, M3+, is
selected from the group consisting of cerium, lanthanum, iron, chromium,
vanadium and cobalt, preferably the trivalent metal is cerium, lanthanum or
iron.
W0 95132797 219 0 7~4 ~ PCTlUS95106323
-- 1 6--
t
J is a monom, i~~~"~~~'elected from the group consisting of VOg,
HV04, V04, Vp07, .W~20~, VgOg, VQOfp, W04, and Mo04; v is the net ionic
charge associated with J; and k is about 0.01 to less than 2.
A is C03, OH, S03, S04, CI, or N03; q is the net ionic charge
associated with A; and m is a positive number.
The calcining is performed for about one hour or more at a
temperature of at least about 450°C, preferably a temperature in the
range of
about 450° to about 1000°, and more preferably a temperature in
the range
of about 450° to about 850°C. The calcining may be pertormed
repeatedly,
in two or more operations preferably interspersed by contacting with an
aqueous metalate salt solution. The final calcining produces a dehydrated
and collapsed composition, as described above.
In still another aspect, the invention is a process for separating sulfur
oxides from a gas containing sulfur oxides which comprises absorbing sulfur
oxides by exposing a gaseous mixture containing sulfur oxides to a
dehydrated and collapsed composition, as described above. Herein the
term "absorbing" is understood to include absorption, adsorption, imbibition,
and chemisorption. The sulfur oxides include sulfur dioxide and sulfur
trioxide. The absorbing causes the sulfur oxide to become associated with
and remain in close proximity to the collapsed composition. It is not entirely
clear whether the sulfur oxides are associated in the form of sulfur trioxide
molecules, an anion containing sulfur and oxide, or some other chemical
species.
The gaseous mixture from which sulfur oxides are removed need not
contain molecular oxygen, but in a preferred aspect of the invention
desirably contains an amount of molecular oxygen which is in excess of the
stoichometric amount required to convert any sulfur dioxide present to sulfur
trioxide. The excess of molecular oxygen need not be large, but the ability of
the absorbent of this invention to absorb sulfur dioxide is improved as the
amount of excess molecular oxygen increases. Although the reason for this
effect by molecular oxygen is uncertain, it is believed that increased
concentrations of oxygen promote the conversion of sulfur dioxide to sulfur
trioxide and assist in rejuvenating an oxidation function of the adsorbent. It
is also believed that this sulfur trioxide is more easily absorbed by the
absorbent than is the sulfur dioxide. The molecular oxygen can either be
inherently present in the sulfur oxide containing gaseous mixture or can be
added. The absorption of sulfur oxides is desirably carried out at a
temperature below about 900°C, preferably at a temperature of about
100°
~'19074~
W 0 95132797 - _ 1, 7 - ' , PCTIUS95106323
to about 900°C and most preferably at a temperature of about
300° to about
800°C. .
The sulfur oxides are desorbed by contacting the collapsed
composition with a hydrocarbon in the presence of a cracking catalyst at an
elevated temperature. The temperature is desirably about 375° to about
900°C, preferably about 430° to about 700°C, and mast
preferably about
450° to about 650°C. Any hydrocarbon can be used to remove the
absorbed sulfur oxides from the absorbent of this invention so long as it can
be cracked by the cracking catalyst at the temperatures employed. Suitable
hydrocarbons include, but are not limited to methane, natural gas, natural
gas liquids, naphtha, light gas oils, heavy gas oils, wide-cut gas oils,
vacuum
gas oils, decanted oils, and reduced crude oils as well as hydrocarbon
fractions derived from shale oils, coal liquefaction and the like; such
hydrocarbons can be employed either singly or in any desired combination.
Additionally, the sulfur oxides can be desorbed by contact with a reducing
gas such as hydrogen.
Although the invention disclosed herein is not to be so limited, it is
hypothesized that a chemical reaction occurs between the collapsed
composition and the sulfur oxides which results in the formation of non-
volatile inorganic sulfur compounds, such as sulfites and sulfates, at
relatively high temperatures. These sulfites and sulfates can undergo partial
decomposition to liberate the original sulfur oxides and collapsed
composition. As a consequence of this reversal of the sulfur oxide
absorption at high temperature, the absorption of sulfur oxides is desirably
effected at a temperature below about 900°C and preferably below about
800°C.
The precise mechanism by which absorbed sulfur oxides are
removed from the collapsed composition is unknown, but it is believed that
the combination of hydrocarbons and hydrocarbon cracking catalyst and
elevated temperatures produces a reducing environment which effects a
conversion of absorbed sulfur oxides to hydrogen sulfide while
simultaneously reactivating the collapsed composition for further absorption
of sulfur oxides. The removal of absorbed sulfur oxides from the collapsed
composition is generally improved by contacting the collapsed composition
with added steam either simultaneously with or subsequent to treatment with
a hydrocarbon in the presence of a cracking catalyst.
The hydrogen sulfide which is produced during the removal of
absorbed sulfur oxides from the collapsed composition can be converted to
~19074:~
W 0 95132797 PCTIUS95/06323
__ig__
elemental sulfur by any of the conv,~tionaf techniques which are well-
known to the art as, for ex~~~e~~ a~Claus Unit. Cracked hydrocarbon
products which are produced during removal of absorbed sulfur oxides can
be recycled for further use in removing absorbed sulfur oxides.
It is highly preferable that the process for separating sulfur oxides
further comprise exposing a layered mixed hydroxide of the formula:
M2X AIZ_n M ~+ ~OH~4x.rd \w/ k 1Aq/ 2-kk ~ m H 20
v q
where M2+ is a divalent metal selected from the group consisting of
magnesium, calcium, and zinc;
x is about 1.1 to about 3.5;
n is about 0.01 to about 0.4;
M3+ is a trivalent metal selected from the group consisting of cerium,
iron, chromium, vanadium and cobalt;
J iS VOg, HVO4, VpO~, HV20~, V3O9, Vq0~2, VO4, WO4 Of MoO4;
v is the net ionic charge associated with J;
k is about 0.01 to less than 2;
A is selected from the group consisting of C03, OH, S03, S04, CI,
NOg;
q is the net ionic charge associated with A; and
m is a positive number,
to the gaseous mixture containing sulfur oxides at a temperature in the
range of about 450° to about 1000°C to produce the collapsed
composition
described above.
Suitable cracking catalyst for use in the practice of this invention
includes all high activity solid catalysts which are stable under the required
conditions. Suitable catalysts include those of the amorphous silica-alumina
type, having an alumina content of about 10 to about 30 weight percent.
Catalysts of the silica magnesia type are also suitable which have a
magnesia content of about 20 weight percent. Preferred catalysts include
those of the zeolite-type which comprise from about 0.5 to about 50 weight
percent and preferably about 1 to about 30 weight percent of a crystalline
alumina silicate compound distributed through a porous matrix. Zeolite-type
cracking catalysts are preferred because of their thermal stability and high
catalytic activity.
~~~~~~~ ; ~;
W O 95132797 - _ 1 9 - _ ' pCTIUS95J06323
The crystalline alumina silicate or zeolite component of the zeolite-
type cracking catalyst can be of any type or combination of types, natural or
synthetic, which is known to be useful in catalyzing the cracking of
hydrocarbons. Suitable zeolites include both naturally occurring and
synthetic alumina silicate materials such as faujasite, chabazite, mordenite,
Zeolite X, Zeolite Y, and ultra-stable large pore zeolites. The zeolite-type
cracking catalyst may be dispersed within a porous refractory material,
natural or synthetic, which.can be, for example, silica, alumina magnesia,
boria, kieselguhr, diatomaceous earth, and mullite.
In the practice of this invention, the collapsed composition can be
incorporated into or deposited onto a suitable support. Suitable supports
include, but are not limited to, amorphous cracking catalyst, zeolite-type
cracking catalyst, silica, alumina, mixtures of silica and alumina, magnesia,
mixtures of silica and magnesia, kieselguhr, kaolin, and diatomaceous earth.
Preferably the support is porous and has a surface area including the area
of the pores open to the surface of at least about 10, preferably at least
about
50, and most preferably about 100 square meters per gram.
A highly preferred aspect of this invention comprises its use to reduce
sulfur oxide emissions from catalyst regeneration in a cyclic, fluidized,
catalytic cracking process. In this embodiment, a fluidizable particulate
solid
which comprises a dehydrated and collapsed composition is circulated
through the fluidized catalytic cracking process in association with a
particulate cracking catalyst. The collapsed composition is of the formula:
2+ 3+
M2m A12-p M P Tr 07+r~s
The collapsed composition is substantially composed of
microcrystallites of about 0.1 to about 30 nanometers. One portion of the
microcrystallites are constituted by a solid solution phase having alumina
oxide dispersed in a divalent metal monoxide crystal structure. Another
portion of the microcrystallites is constituted by a spinet phase.
Additionally,
microcrystallites constituted by a trivalent metal oxide phase may be present.
The phases are discernible by high resolution electron microscopy.
A feedstock is mixed with a particulate cracking catalyst in a reaction
zone at a temperature in the range of about 430°C to about 730°C
to
produce cracked hydrocarbons. The contact with the cracking catalyst is
preferably effected in one or more fluidized transfer line reactors at
cracking
2~90~45
WO 95!32797 _ _ 2 O _ _ PCT/US95106323
temperature and at a fluidizing velocity which limits the cracking time to not
more than about 10 seconds. Reaction zone effluent, comprises
hydrocarbon vapors both cracked an~ uncracked, cracking catalyst and a
carbonaceous material referrexd~d~e~coke which contains sulfur, relatively
volatile carbonaceous cortfRchnents and relatively less volatile carbonaceous
components. A significant proportion of the coke adheres to the cracking
catalyst.
The carbonaceous components of coke comprise highly condensed
aromatic hydrocarbons which generally contain a minor amount of
hydrogen, generally from about 4 to about 10 weight percent of hydrogen.
Wheri the hydrocarbon feedstock contains organic sulfur compounds, the
coke also contains sulfur. As the coke builds up on the cracking catalyst, the
activity of the catalyst for cracking and the selectivity of the cracking
catalyst
diminishes. The catalyst can, however, recover a major portion of its original
capabilities by a suitable regeneration process.
Hydrocarbon vapors are separated from the cracking catalyst, and the
cracking catalyst is stripped of volatile deposits before regeneration. The
stripping zone can be suitably maintained at a temperature in the range of
about 430°C to about 700°C, preferably about 450° to
about 650°C and
most preferably from about 765° to about 595°C. The preferred
stripping
gas is steam although inert gases, such as nitrogen or flue gases, or
mixtures of steam with inert gases can also be used.
Stripped and partially deactivated cracking catalyst is regenerated by
burning the coke deposits from the catalyst surface with a molecular oxygen
containing regeneration gas, such as air, in a regeneration zone. This
burning results in the formation of combustion products such as sulfur
oxides, carbon monoxide, carbon dioxide and steam. The oxygen
containing regeneration gas can contain relatively inert gases such as
nitrogen, steam, carbon dioxide, recycled regeneration zone effluent and the
like. The molecular oxygen concentration of the regeneration gas is
ordinarily from 2 to about 30 volume percent and preferably from about 5 to
about 25 volume percent. Since air is conveniently employed as a source of
molecular oxygen, a major portion of the inert gas can be nitrogen. The
regeneration zone temperatures are ordinarily in the range of about
565° to
about 790°, and are preferably in the range of about 620° to
about 735°.
The cracking catalyst is then returned to the reaction zone. The
process also comprises removing the absorbed sulfur oxides from the
particulate solid by exposing the particulate solid to the hydrocarbon
~.' ~-~,~.~.
W095132797 --21 --~ t ' a , PCTIUS95106323
feedstocks in the reaction zone. This is conveniently accomplished by
recirculating the particulate solid to the reaction zone along with the
regenerated cracking catalyst.
During the catalytic cracking of the hydrocarbon feedstock in the
reaction zone absorbed sulfur oxides are substantially released from the
particulate solid as sulfur containing gas comprising hydrogen sulfide.
Similarly, subsequent steam stripping serves not only to remove the volatile
coke components from the cracking catalyst, but also serves to complete the
removal of any residual absorbed sulfur oxides from the particulate solid and
complete the reactivation of the collapsed composition for further absorption
of sulfur oxides in the regeneration zone. The resulting hydrogen sulfide is
recovered together with other products from the reaction zone, and stripping
zones and can be converted to elemental sulfur in facilities which are
conventionally associated with a fluidized catalytic cracking unit.
The following Examples are not intended to limit the scope of the
invention in any manner but, rather, are presented in order to better
communicate certain aspects of the invention.
Example 1: Preparation of a Layered Mixed Hydroxide
One liter of deionized water, 15.90 grams (0.15 mol) of sodium
carbonate, and 48.0 grams (1.2 mol) of sodium hydroxide were charged to a
flask equipped with a mechanical stirrer and a water-cooled reflux
condenser. One liter of deionized water, 102.56 grams (0.4 mol) of Mg(N03)
2.6H20, 67.38 grams (0.1796 mol) of AI(NOg) 3.9H20, and 8.86 grams
(0.0204) of Ce(N03) 3.6H20 were blended and added dropwise to the flask
with continuous stirring over a period of about one hour. The result was a
gelatinous mixture of 10.74 pH which was heated under reflux while being
swept with a nitrogen purge for about 15 hours at 85°C. The mixture was
subsequently cooled, filtered, washed repeatedly with deionized water, and
dried overnight under vacuum at 70°. The dried material was designated
Sample A.
Sample A was analyzed by conventional x-ray diffraction techniques
which produced the trace presented in Fig. 1. The trace includes peaks
characteristic of a hydrotalcite structure having a d(001 ) value of 7.62
Angstrom units. Additionally, Sample A was analyzed for metals by
inductively coupled plasma techniques and the metals were reported as
W095132797 ~ ~ ~ ~,~~, ~ '~ ~ PCTIUS95106323
20.6 percent magnesium, 12.0 percent aluminum, 6.1 cerium, and 450 ppm
sodium. The reported metals correspond to a hydrotalc'tte clay having the
formula:
M94A12.099C80.205(~H)12.912C~3~4 I"120
Example 2: Calcination of a Layered Mixed Hydroxide at 450°
A portion of the dried material produced by the procedure described
in Example 1 above was heated in air at a rate of about 20°C per minute
until a temperature of 450°C was achieved. The material was held at
450°
for 15 hours and then cooled. The material calcined at 450° was
designated
Sample B.
Example 3: Calcination of a Layered Mixed Hydroxide at 850°
A portion of the dried material produced by the procedure described
in Example 1 above was heated in air at a rate of about 20°C per minute
until a temperature of 850° was achieved. The material was held at
850° for
15 hours and then cooled. The material calcined at 850° was designated
Sample C.
Example 4: Vanadation of 450° Calcined Material
A solution was prepared by blending 0.23 grams of NH4VO3 with 6.22
grams of deionized water. The proportions of the solution had been
carefully chosen to provide a pH in the range of about 6 to about 8 in order
to stabilize metavanadate ions (V031~). The solution was thoroughly mixed
with 4.82 grams of Sample B which was described in Example 2 above. The
resulting mixtures was dried under vacuum at 70°C overnight and then
calaned at 450° to produce a vanadated material designated Sample D.
Based on the proportions of reactants, it is estimated that the theoretical
formula of Sample D is approximately:
M9aA11.79sCeo.2o4VO.o7407.185
Subsequent analyses of Sample D were reported as B.E.T. surface
area of 192 m2lg, average pore radius of 81 Angstrom units, micropore area
of 50 m2/g, and micropore volume of 0.023 cdg.
''r , l i5 4.
, l . . d '
WO 95!32797 _ _ 2 3 _ _ PCTIU595/06323
Example 5: Vanadation of 850° Calcined Material
The procedure of Example 4 was performed again except that
Sample C described in Example 3 above was vanadated and calcined. The
resulting material was designated Sample E. The estimated theoretical
formula for Sample E is identical to the formula presented above for Sample
D.
Subsequent analyses indicated that Sample E had a B.E.T. surface
area of 119 m2lg, an average pore radius of 91 Angstrom units, a micropore
area of 6 m2/g, and a micropore volume of 0.003 cc/g. High resolution
electron photomicrogaphy of Sample D, supported by scanning electron
microscope analyses, indicated that a significant fraction of the
microcrystallites present were composed of a solid solution phase having
aluminum oxide dispersed in a magnesium oxide crystal lattice.
Example 6: Reactant Limited Vanadation
A relatively dilute vanadate solution was prepared by blending 0.11
grams of NH4V03 with 4.0 liters of deionized water. The proportions of the
solution had been chosen to provide a pH in the range which stabilizes the
metavanadate form of the anion. The dilute solution was thoroughly mixed
with 4.82 grams of Sample B which was described in Example 2 above. The
resulting mixture was filtered, then dried under vacuum at 70°C
overnight
and calcined at 450° to produce a partially vanadated material.
WO 95132797 ~ Z~'~ _ _ 2 4 _ _ PCTIUS95106323
Based on the proportions of reactants, it is estimated that the
theoretical formula of the partially vanadated material is approximately:
M94p'l1.796~'e0.204V0.035~7.088
Example 7: Performance Testing of Sample D . ' ,
A portion of Sample D which was described in Example 4 above was
subjected to thermal gravimetric analysis during sequential periods of
exposure to an oxidizing gas mixture including 5000 ppm sulfur dioxide, 2
percent oxygen and balance helium and to a reducing gas mixture including
50 percent hydrogen and balance helium. The temperature of the oxidizing
gas was about 735°C, while the temperature of the reducing gas was
about
640°C. Each exposure was preceded by a period of helium purge at the
corresponding temperature.
The sample, which weighed l 9.7363 milligrams at the outset,
exhibited a relative weight increase of 84.6 percent during ninety minutes of
exposure to the oxidizing gas mixture An almost immediate relative weight
decrease of 87.3 percent was observed on exposure to the reducing gas
mixture. Figure 2 depicts the relationship of relative sample weight,
expressed as a percentage of the weight at the outset, to elapsed time in
minutes for one oxidizing and reducing cycle of the thermal gravimetric
analysis for Sample D.
Example 8: Performance Testing of Sample E
The procedure of Example 7 above was repeated with a portion of
Sample E which was described in Example 5 above. The sample weighed
20.8923 milligrams at the outset. The sample increased in relative weight by
83.3 percent while exposed to the oxidizing gas mixture, and decreased in
relative weight by 84.1 percent weight during the exposure to the reducing
gas, based on the weight at the outset. Figure 3 shows the relationship of
relative sample weight, expressed as a percentage of the weight at the
outset, to elapsed time in minutes for one oxidizing and reducing cycle of the
thermal gravimetric analysis for Sample E.
Some of the Sample E material subjected to thermal gravimetric
analysis was also tested for thermal gravimetric performance during a
W095132797 _ _ 2 5 _ _ . PCTIUS95/06323
second oxidizing and reducing cycle and a third oxidizing and reducing
cycle. The sample weight at the outset of the second cycle was 16.8988
milligrams, with the third cycle following immediately after the second.
Relative sample weight as of a function of elapsed time as well as a
temperature trace for the second and third cycles of Sample E is presented
graphically in Fig. 4.
Example 9: Performance Testing of a Spinet Absorbent
A widely used sulfur oxide absorbent based on a spinet composition
was commercially obtained and designated Sample F for the purpose of
performing a control experiment. Sample F is not of the present invention.
However, Sample F was subjected to one oxidizing and reducing cycle of
the performance test described in Example 7 and Example 8 above.
Sample F exhibited a 47.0 percent increase in relative weight over a period
of 90 minutes exposure to the oxidizing gas containing sulfur dioxide.
Exposure to the reducing gas containing hydrogen caused an almost
immediate relative weight decrease of 50.3 percent.
The results of pertormance testing produced in Example 7, Example 8
and Example 9 are presented in Table I, below.
TABLE I
Absorbent Cycle Relative Weight Time Relative Weigh Time
Sample Increase (percent) rains. Decrease (percent) rains.
D 1 84.6 90 87.3 rapid
E 1 83.3 90 84.1 rapid
2 76.2 90 77.4 rapid
3 78.3 90 not studied -
F 1 47.0 90 50.3 rapid
Inspection of Table I reveals that the absorbents prepared according
to the present invention, Sample D and Sample E, demonstrated a
significantly larger capacity for sulfur dioxide absorption than did the
control
sample, Sample F. Additionally, Sample E of the present invention
continued to absorb more sulfur dioxide on its second and third oxidizing
cycles than did control Sample F on its initial oxidizing cycle. The results
are
especially surprising because Sample F is a commercially obtained
~190~45
WO 95132797 _ _ 2 6 _ _ PCTIUS95/06323
>.
absorbent which is representative of widely accepted and currently utilized
absorbent technology.
For the purposes of t~h~~p,~esent specification, "predominantly" is
defined as mostly or rrt~f"'e~~n than not. In quantative terms, predominantly
denotes about 50 pe~rc;ent or more. "Substantially" is defined as being
present in significant proportions or having sufficient frequency so as to
measurably affect macroscopic qualities of an associated compound or
system. Where the amount required for such significant and measurable
impact is not clear, substantially is synonymous with about 20 percent or
more. "Essentially" is defined as absolutely but allowing for some small
variations which have a negligible effect on macroscopic qualities and final
outcome. Variations of about one percent can often exist without any
detectable change in essential qualities.
Examples have been presented and hypotheses advanced in order to
better communicate certain facets of the invention. The scope of the
invention is determined solely by the appended claims, and is not limited in
any way by the examples or the hypotheses. Moreover, practitioners who
study the teachings set forth above will undoubtedly receive suggestions
which bring to mind many additional aspects of the invention. Such
obviously similar aspects, whether or not expressly described herein, are
intended to be within the scope of the present claims.
